1. Field of the Invention
The present invention relates to an optical device mounted upon a floating assembly, which is floated above an information recording medium, for optically recording the information upon the recording medium and reproducing the information recorded thereupon.
2. Description of the Prior Art
In magnetic recording and reproducing systems using magnetic disks, there has been known a floating magnetic head which floats at an extremely small distance above a rotating magnetic disk by the air flows produced along the surface thereof. An air bearing assembly for floating the magnetic head in the manner described above is generally known as the dynamic pressure type floating assembly, and finds a wide range of applications in magnetic disk devices used as peripheral equipment of electronic computers and in video disk devices for recording and reproducing the still TV images. The lift for floating the dynamic pressure type floating assembly and devices mounted thereupon is, in general, produced by a floating shoe or shoes. The surface of the floating shoe in opposed relation with the upper surface or recording surface of the disk is curved so that the lift is produced by the dynamic pressure of the air flowing between the shoe and the disk. The floating height, that is, the distance between the floating assembly and the upper surface of the disk, is varied in proportion to the relative speed between the floating shoe and the disk. The floating assembly of the type described is disclosed in "A Gas Film Lubrication Study", IMB Journal, July, 1959.
Recently there has been and demonstrated the optical disk device of the type in which the information is optically recorded upon a recording medium and is reproduced therefrom, and various studies and experiments have been made in order to develop a floating optical head capable of floating at a predetermined height above a rotating optical recording disk for an optical recording and reproducing system which is considerably heavier than the conventional magnetic heads. The underlying principle of the floating optical heads is substantially similar to that of the floating magnetic heads, but it is essentail for the floating optical heads that the recording surface must be maintained within the depth of field or focus of the optical head.
In general, the linear speed of the optical disk or disk-shaped optical recording medium, is faster at a point closer to its outer periphery than at a point closer to its inner periphery or center. As a result, the floating height of the floating assembly whose lift is dependent upon the velocity of the air flow, changes depending upon the position of the floating assembly along the radial line of the optical disk along which is displaced the floating assembly or optical head. Consequently, the relative distance between the optical system mounted upon the floating assembly and the surface of the disk changes depending upon the position of the floating head along the radial line of the disk. Therefore, when the optical system with a fixed focal distance is used, the light beam is not correctly focused upon the surface of the disk. That is, the sharp optical patterns are not recorded in writting while in reproduction the signal with a low S/N ratio is reproduced. The above problem also occurs when the rotational speed of the disk changes.
In general, there has been so far employed the method for exerting the force by a spring or the like to the floating assembly in the direction opposite to the direction in which the air flow strikes against the floating assembly in order to maintain the floating height of the floating assembly constant. In this case, the floating height is determined by the balance or equilibrium between the lift and the force exerted to the floating assembly toward the disk. Therefore, the downward force exerted to the assembly must be suitably changed depending upon the position of the floating assembly in order to compensate the variation in relative speed between the disk and the floating assembly. For this purpose, there has been demonstrated a method in which the position of the floating assembly above the disk along the radial line thereof is detected by a cam mechanism, and the downward force exerted to the floating assembly by a spring or the like is controlled by a mechanical linkage in response to the output of the position detecting cam mechanism so that the light beam may be always focused upon the surface of the disk.
However, in addition to the lift, the external force or the downward force is exerted to the assembly so that its position and operation become unstable. In some case, the floating assembly is too much inclined so that it collides with the surface of the disk. Furthermore, it is extremely difficult to determine where the downward force must be applied and the magnitude thereof. Moreover, the deflection of a spring which is used for exerting the downward force to the floating assembly is limited. Therefore, the force of the spring changes as its deflection changes when the floating assembly follows a relatively greater wrap or ondulation of the disk so that the floating height cannot be maintained constant. Thus, the controllable range of the floating assembly is limited when the spring is used. Same is true when the floating height changes due to the change in rotational speed of the disk.